Guess I found the right configuration after following this guide:
https://docs.opnsense.org/manual/how-tos/wireguard-selective-routing.html
I had to disable "Dynamic gateway policy" on the Wireguard Interface.
Instead a gateway IP has to be added to the Wireguard VPN instance.
Afterwards a new gateway needs to be configured on System settings as mentioned above.
Seems to work even after a restart now.
https://docs.opnsense.org/manual/how-tos/wireguard-selective-routing.html
I had to disable "Dynamic gateway policy" on the Wireguard Interface.
Instead a gateway IP has to be added to the Wireguard VPN instance.
Afterwards a new gateway needs to be configured on System settings as mentioned above.
Seems to work even after a restart now.
"